This invention relates to an improved hydraulic or hydromechanical transmission in which the transmission ratio is infinitely variable between a neutral condition and a direct drive condition.
Such transmissions are used in a wide variety of vehicles and other equipment which require changes in the ratio of input shaft speed to output shaft speed. The known prior art in this field is exemplified by the U. S. Pats to Lee Nos. 1,648,000, Creighton et al. No. 3,131,539, and Reimer No. 3,698,189. In each of these patents, a hydraulic pump unit and cooperating swash plate on the input side of the transmission interact to displace hydraulic fluid to an outside pump which in turn interacts with an output side swash plate in a manner to rotate the output shaft. The transmission ration essentially depends on the fluid displacement ratio of the two pumps.
While transmissions and torque converters of this type have been generally satisfactory in many applications, they have not been wholly free of difficulty. One major problem has been to provide a reliable means for smoothly and accurately adjusting the transmission ratio throughout a continuous range. This is normally done by varying the angle at which the swash plates are tilted and thus to change the relative displacement capacity of the two pumps. The main disadvantages of the conventional shifting arrangment is that the swash plates must be mounted in a manner to permit them to be tilted and held at varius angles. Of course, the necessity for mounting the swash plates such that they are tiltable significanly increases the complexity and cost of the transmission, as well as adding substantially to the maintenance requirements. Moreover, complicated mechanical linkages are required to move the swash plates in a manner to accurately adjust their tilt angle throughout a continuous range. Such tilting movement of the swash plates involves additional practical problems because considerable fluid pressure must be overcome and jerkiness in the shifting often results.
Other problems have been caused by the uneven torque conditions which are present in existing transmissions of this type. Since each swash plate typically presents a cam surface that lies in a single inclined plane, the forces caused by the pumping and driving action at any given time are concentrated on one side of the transmission axis, thus subjecting the shafts to excessive stress due to the uneven torque forces. For the same reason, the hydraulic fluid is distributed non-uniformly about the transmission axis, and its weight adds significantly to the unbalanced condition of the transmission. Manifestly, the uneven torque effects and unbalanced weight result in excessive wear on the shafts and other parts of the transmission.
Existing hydraulic transmission of this type are further characterized by a large number of parts which rotate relative to one another. Typically, the parts are located in a stationaly housing with the two pumps being rotatable relative to one another and the two swash plates being rotatable relative to one another and to the pumps. Accordingly, a large number of bearing, bushings, dynamic oil seals and the like are required, with corrresponding increases in the cost, complexity, and maintenance problems. It has not been possible in the past to provide transmission housings which rotate with the shafts because of the unbalanced weight problems previously pointed out.
It is primary object of the present invention to provide a hydraulic transmission which is improved over existing transmissions as to its construction and operating characteristics.
More specifically, an object of the invention is to provide a hydraulic transmission in which the transmission ratio may be varied continuously between a neutral condition and a direct drive, one to one correlation of input shaft speed to output shaft speed.
In conjunction with the preceding object, it is another object of the invention to provide an improved mechanism for adjusting the transmission ratio, which mechanism may be easily operated either manually or in automatic fashion such as electrically, hydraulically, or by means of vacuum. In this respect, the control sleeve and lever arrangement is of considerable significance in that it eliminates the necessity of mounting the swash plates for adjustable tilting as is done in the known prior art transmissions.
Still another object of the invention is to provide a hydraulic transmission which is uniquely constructed in order to achieve a balance of the torque forces and the fluid distribution with respect to the transmission axis. Such balancing is achieved through the use of swash plates which are shaped symmetrically with respect to the shafts.
Yet another object of the invention is to provide, in a hydraulic transmission of the character described, a variable displacement fluid pump in which the pumping capacity is varied by restricting the stroke length of the pumping elements.
A further object of the invention is to provide a hydraulic transmission of the character described in which virtually no pumping action or fluid displacement occurs at the direct drive condition. Accordingly, friction and other wear on the components is reduced since there is no relative motion between the parts at the high or one to one setting.
An additional object of the invention is to provide a hydraulic transmission which may be constructed either with both pumps interlocked for rotation in unison, or with both swash plates rigidly coupled for rotation together. In both embodiments, the number of parts that rotate relative to one another is reduced in comparison to existing transmissions, and the number of bearings and seals required is thus decreased.
Yet another object of the invention is to provide a hydraulic transmission of the character described which has a housing that rotates along with the remaining components, unlike prior art transmissions which have a stationarily mounted housing.
Still further object of the invention is to provide a hydraulic transmission which is relatively simple to assembly, which requires little maintenance, and which functions reliably over a long operating life.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.